High Quality Content by WIKIPEDIA articles! The Bell P-39 Airacobra was one of the principal American fighter aircraft in service when the United States entered World War II. It was the first fighter in history with a tricycle undercarriage and the first to have the engine installed in the center fuselage, behind the pilot. Although its mid-engine placement was innovative, the P-39 design was handicapped by the absence of an efficient turbo-supercharger, limiting it to low-altitude work. The P-39 was used with great success by the Soviet Air Force, who scored the highest number of individual kills attributed to any U.S. fighter type. Other important users were the Free French and co-belligerent Italian air forces. Together with the derivative P-63 Kingcobra, these aircraft became the most successful mass-produced fixed-wing aircraft manufactured by Bell.